1. Field of the Invention
The present invention relates generally to data erasing and re-writing circuits for use in a microcomputer integrated circuit device. More particularly, the present invention relates to a data erasing and re-writing circuit for use in a microcomputer integrated circuit device which circuit erases and re-writes a program of a memory contained in an integrated circuit referred to as a one-chip microcomputer.
2. Description of the Background Art
In resent years, there is a demand for a microcomputer with a programmable, and erasable and programmable EPROM connected thereto and such microcomputer has come into practical use.
FIG. 6 is a schematic block diagram showing a conventional microcomputer integrated circuit containing an EPROM without a representation irrelevant to the present invention. A CPU1 is connected through a bus 5 to an erasable and programmable read only memory (referred to as an EPROM hereinafter) 2 and an input/output port 3, both of which receive external input/output signals. A write control circuit 4 is provided for writing the data of the input/output signals to the EPROM 2. The circuit 4 receives external write control signals.
Operation of the one-chip microcomputer shown in FIG. 6 will be described. The CPU1 executes a program written in the EPROM 2 and refers to the data written in the EPROM if necessary. The external input/output signals are received and sent through the input/output port 3. In response to an external write control signal, the write control circuit 4 writes data in the EPROM according to address signals included in the input/output signals. The EPROM 2 is programmed by using a generally-called EPROM programmer device before mounting the one-chip microcomputer device on a printed circuit board.
Thus arranged conventional microcomputer requires a control program to be written in an EPROM by another device before packaging on a printed circuit board. For rewriting a once written program, the storage contents of the EPROM 2 with a transparent window should be erased through ultra-violet light irradiation and the EPROM 2 should be re-programmed before packaging thereof. In particular, a surface mounting type package referred to as a multi-terminal flat pack type package causes damage to lead lines in an integrated circuit or a pattern on a printed circuit board when in taking off the package from the printed circuit board or re-mounting the same, thereby making it difficult to re-writing program in the practice.
In addition, at the time of programming the EPROM when the computer device is mounted on a printed circuit board on which input/output port 3 is OR-connected to external circuits through wires, the external circuits might be short-circuited to produce excessive load.